Python 2 compiler.ast replacements for Python 3, functions Discard and Const

Question

I am trying to convert an older library that maps dependencies called snakefood over to Python 3. I have 2 parts I can't figure out what to substitute as I can't find documentation on what either function does - from compiler.ast import Discard, Const I can't seem to find an equivalent in Python 3 in the ast library. Here are the function calls from Python 2, see both here being used in isinstance() calls:

    def default(self, node):
        pragma = None
        if self.recent:
            if isinstance(node, Discard):
                children = node.getChildren()
                if len(children) == 1 and isinstance(children[0], Const):
                    const_node = children[0]
                    pragma = const_node.value

        self.accept_imports(pragma)

Apologies for not understanding this stuff, I just learned about AST calls trying to use this library. Much appreciated.

anthony sottile · Accepted Answer · 2017-07-18 19:04:58Z

1

Discard (took me a while to figure out what it does) is now Expr (though this includes more things than previously)

I got this hint from the source of compiler/codegen.py:

    def visitDiscard(self, node):
        # XXX Discard means it's an expression.  Perhaps this is a bad
        # name.

Const has been replaced with several different types which represent various constants, notably Num, Str, JoinedStr, and a few others. The documentation for all the ast types can be found here.

answered Jul 18, 2017 at 19:04

anthony sottile

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3 Comments

jasonharper Over a year ago

So a Discard node that contains only a Const child would be something like a docstring.

anthony sottile Over a year ago

@jasonharper I also see it for numbers: >>> compiler.parse('2') Module(None, Stmt([Discard(Const(2))]))

Dan Locks Mar 13 at 19:59

From the documentation for 3.8, different types of constants are now all represented by Constant. docs.python.org/3/library/ast.html

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